In recent years, development in Information Technology has been remarkable. The frequencies handled by communication devices move to higher frequencies from day to day, and even frequencies in millimeter waves band are utilized. Until recently, compound semiconductors having good insulation properties have been predominant radio frequency semiconductors.
However, advance of the scaling down in recent years has enabled transistors using conductive silicon-based substrates to obtain frequency bands in radio frequency close to those of the compound semiconductors. Moreover, the transistors using silicon-based substrates can be manufactured at lower cost than the compound semiconductors, and thus are expected to become increasingly popular from now on.
Most challenging to mount a radio frequency front-end and a digital circuit on one chip is to implement a power amplifier that employs an MOS (Metal Oxide Semiconductor) transistor. The advance of scaling down a silicon in recent years has improved the frequency bands in radio frequency. However, an oxide layer for use as a gate insulating film of the MOS transistor is extremely thin and thus the withstand voltage is low. For example, while the withstand voltage of several tens of volts is achieved in the compound semiconductors, the withstand voltage of merely up to about 2 V is achieved in the MOS transistors using silicon that are for use at radio frequency.
As described above, a sufficient withstand voltage is not obtained by one MOS transistor. To obtain high output power, it is necessary to combine outputs of a plurality of transistors. As a method for combining the outputs of a plurality of transistors, various combiners or power amplifiers have been proposed, examples of which include a Wilkinson-type power divider/combiner and the power amplifier disclosed in PTL 1.
PTL 1 discloses a technology known generally as a distributed active transformer (DAT). FIG. 1 is a diagram showing a configuration of a conventional power amplifier 10.
As shown in FIG. 1, the power amplifier 10 disclosed in PTL 1 includes a plurality of push-pull amplifiers 21, a plurality of slab inductors 22, and a metallic coil 23. The plurality of push-pull amplifiers 21 are interconnected annually via the plurality of slab inductors 22. The plurality of slab inductors 22 functions as primary windings, and the metallic coil 23 functions as a secondary winding.
The power amplifier 10 disclosed in PTL 1 further includes a spiral transformer balun 30, differential lines 40 and 41, and a distributed network 50, to match input impedance. The distributed network 50 provides, to the gates of transistor pair included in the push-pull amplifier, a balanced input signal inputted via the spiral transformer balun 30 and the differential lines 40 and 41.